There is a need for thin crystalline ribbons and films of many materials such as silicon and other semiconductors. These ribbons are often very costly and difficult to produce. For example, thin wafers of monocrystalline semiconductor materials are generally produced from monocrystalline boules grown by the Czochralski technique. The preparation of the thin wafers from large crystal boules requires slicing and polishing, is a costly and time consuming technique, and inherently wastes much of the boule. Consequently, much effort has been directed toward growing thin monocrystalline ribbons that need only be scribed and broken to be used.
Ribbons of semiconductors have been produced by pulling substantially in a horizontal plane from a melt surface, as disclosed by Bleil in U.S. Pat. Nos. 3,681,033 and 3,759,671. According to those patents, crystal ribbons are grown from a melt in a crucible wherein a heater is submersed below the melt surface, a heat sink is positioned above the melt surface, and the ribbon is pulled horizontally from the surface of the melt. This technique produces ribbons much faster than the previous methods. However, pulling a ribbon too quickly or at too great an angle from horizontal introduces grain boundaries and imperfections which degrade the performance of circuitry placed on the semiconductor surface. Moreover, the necessary controls to implement the process and produce very thin crystal ribbons and films of good quality are difficult to manage and thus the commercial advantage is reduced.
In some applications, it is desirable to prepare monocrystalline ribbons or films of semiconductor or other materials on insulator substrates, such as semiconductor on insulator, or SOI, structures. This can be accomplished by growing the ribbon on the insulator or bonding the ribbon onto insulator material by using, for example, electro-bonding techniques. The desired monocrystalline structure can be given to the ribbon material through a variety of "seeding" techniques, including beginning and maintaining the seeding process at a location away from the insulator material, thereby inducing a monocrystalline form to be propagated through the ribbon cross section.
For ribbons or SOI structures, a variety of methods for supplying the zone-melting energy have been used, including lasers and graphite heaters with energy-focusing means. Methods for inducing electrical currents in the semiconductor and other materials, by exposing them to a high frequency electric field, have been used to recrystallize cylindrical boules in a particularly energy efficient manner. However, induction methods have only recently been applied to ribbons in a manner permitting control of the shape and size of the zone of recrystallization as disclosed by Bleil in U.S. Pat. Nos. 4,749,438, Method and Apparatus for Zone Recrystallization, 4,775,443 Method and Apparatus for Zone Regrowth of Crystal Ribbons from Bulk Material, and 4,873,063, Apparatus for Zone Regrowth of Crystal Ribbons. These patent disclosures are incorporated herein by reference. It is taught to use capacitive current induction where it is desired to avoid direct contact between the material and the electrodes and to cause zone recrystallization in a thin layer of the material in a manner allowing the shape and size of the zone to be controlled. On the other hand, Rayleigh-Taylor instabilities may be incurred by spacing the electrode from the material to be melted.
The recent Bleil patents further reveal that through proper placement of appropriately shaped electrodes near the surface of a thin layer of material which exhibits sufficient electrical conductivity near its melting point, heating by means of induced electrical currents can produce desirable recrystallization zone sizes and shapes. According to one embodiment, two sets of sheet electrodes are used. A first set of electrodes, near the surface of the ribbon and separated in the pulling direction, induces electrical currents within a specific portion of the layer that are sufficient to raise the material to very nearly its melting temperature. A second pair of electrodes induce enough additional currents to cause a narrow region within the specific portion of the material to melt along a selected stable zone perpendicular to the pulling direction. Proper control of the two currents induced, in coordination with means for replenishing the melt zone and a means such as heat pipes for removing the heat supplied by the induction currents, allows the recrystallization to take place in a desired fashion, at an acceptably high pulling rate.
While ribbon growth has been demonstrated for a given material using the techniques disclosed in these patents, it has become apparent that improvements in the control of the process would lead to more reliable operation, extension to other materials and a better product. For example, there is a tendency for dendritic structures to form in the ribbon. It has been suggested that a low axial temperature gradient in the ribbon is necessary to prevent dendritic growth (P. D. Thomas and R. A. Brown, J. Crystal Growth 82, 1-9 (1987)). As a further example, the first set of electrodes for raising the material to near its melting temperature is limited in its thermal control capability and it is desired to enhance that function by improved temperature control techniques including accurately controlled cooling as well as heating.